add macro command

[NicholasLinderWood]

Adds a new <esc> command :m <macro_name> which inserts the text in the file ~/.config/babi/macros/<macro_name> if it exists.

A couple of notes:

• During our discussion in Optional Template for New File - Feature Request #360 you mentioned calling the command :m for "macro" but entitled the config directory "snippets". At least for me, I thought it made more sense to use "macros" instead of "snippets" (to be consistent with the naming of the command). If you feel otherwise it's obviously fine to go back to calling these snippets, but wanted to give my two cents.
• I don't currently have any status update if loading the macro is successful. If you'd like that I can add it as well.

resolves #360

[asottile]

looks pretty good -- nice and simple and works well -- a few things though:

• this should probably get its own dedicated function in file.py with an appropriate @edit_action etc. decorators (right now if you undo a macro it says undo: uncut selection)
• it should probably post a status after the macro itself is applied -- just something simple

[NicholasLinderWood]

I believe all of these have been addressed! Please let me know if there is anything else! :)

[asottile]